In a typical borehole, a drilling fluid is pumped from the surface to the drill bit through a passage formed in the drillstring; the drilling fluid flows back to the surface within the space surrounding the drillstring. Most drilling operations use "mud" as the drilling fluid, due to its relatively low cost, readily controlled viscosity, and other desirable characteristics. The mud clears the material cut by the drill bit from the borehole and maintains a substantial hydrostatic pressure at the depth of the drill bit that withstands the pressure produced in the surrounding formation. It also lubricates the drillstring and drill bit and seals cracks and crevices in the surrounding formation. However, conventional rotary drilling is slowed by the confining pressure exerted by a column of mud in the borehole. The bottom hole pressure in a hole drilled for oil or gas is typically maintained at a value that is equal to, or slightly greater than, the pore pressure of fluids (water, oil or natural gas) in the formation being drilled. The confining pressure of the mud increases the strength and plasticity of rock, reducing the efficiency of indentation and shear cutting. The greatest effect of confining pressure occurs in shale, which is the most common type of rock encountered while drilling for oil and gas.
It has been demonstrated that significant increases in drilling rate can be achieved by maintaining a borehole pressure that is less than the formation pressure (in a technique referred to as "underbalanced drilling"). Underbalanced drilling is achieved by reducing the amount of weighting material added to the drilling mud or by using gas or foam for the drilling fluid. The problem with underbalanced drilling is that the entire open section of the hole is subject to low pressure, which reduces borehole stability and increases the risk of a "gas kick." Gas kick occurs when the drill bit breaks into a region of higher gas pressure, causing gas bubbles to be entrained in the mud and rise toward the surface; the bubbles expand in volume as the pressure to which the bubbles are exposed drops when the bubbles rise in the borehole.
An ideal hydraulic system would use a low-pressure region that is limited to the bottom of the borehole, with normal pressure controlling formation pressures higher up the hole. There have been attempts to achieve this condition using reverse flow bits; however, the bottom hole pressure reductions achieved with such bits have been relatively minor, i.e., less than 200 psi. Clearly, it would be desirable to create much greater pressure reductions at the bottom of the borehole, to increase drilling efficiency.
The prior art recognizes that it may be desirable to control the flow of drilling fluid within a borehole to improve drilling efficiency. For example, U.S. Pat. Nos. 5,009,272 and 5,190,114 disclose flow pulsing apparatus for a drillstring that includes a valve disposed just upstream of the drill bit. The valve provides a Venturi passage through which the drilling fluid flows to produce a low pressure that actuates either a flap or rolling element to close off the flow of drilling fluid through the valve. Once the flow of the drilling fluid is interrupted, the pressure of the drilling fluid forces the valve open again. The pressures in the valve thus repetitively cycle it between an open and closed state. Drilling mud is water based and is thus substantially incompressible. Each time that the valve closes, the interruption of drilling fluid flow produces a "water hammer" pressure pulse upstream of the valve, due to the inertia of the flowing incompressible fluid against the closed valve. By continually cycling the valve between its open and closed positions, a vibrating force is applied to the drill bit by the repetitive water hammer pulses. However, because the valve in these prior art patents completely interrupts the flow of the drilling fluid through the drillstring to generate the water hammer pulses, it cannot be used with down-hole fluid motors (driven by the flowing drilling fluid), which are often used to rotate drill bits in boreholes, especially those in which the drill bit is at the end of a continuous flexible conduit. Use of this prior art valve is therefore limited to drillstrings comprising coupled sections that are driven by an above-ground motor. Although the interruption of the flow of the drilling fluid by the valve described in these two prior art patents may generate a slight pressure drop at the drill face, the magnitude of this pressure drop is relatively low and does not substantially contribute to an improved drilling efficiency. It would be preferable to generate suction pulses having a magnitude greater than 1000 psi over the entire surface of the drill bits, since pressure pulses at these levels can weaken rock in the formation through which the drill bit is advancing and will greatly improve the efficiency of the drill bit by drawing it into the formation with substantially higher force.
As will be discussed in much greater detail below, suction pressure pulses have other applications besides enhancing the efficiency of the drilling process. Yet, the prior art does not disclose any mechanism to generate suction pressure pulses having a substantial magnitude, and does not disclose or suggest any application for suction pressure pulses.